$4\pi$-periodic Josephson supercurrent in HgTe-based topological Josephson junctions

TL;DR

This paper reports the experimental observation of a $4 extpi$-periodic supercurrent in HgTe-based topological Josephson junctions, indicating the presence of topologically protected gapless Andreev bound states.

Contribution

It provides the first experimental evidence of a $4 extpi$-periodic supercurrent in topological insulator Josephson junctions, linking it to gapless Andreev states.

Findings

Observation of anomalous rf response in HgTe junctions

Evidence of $4 extpi$-periodic supercurrent contribution

Compatibility with theoretical predictions of topological states

Abstract

The Josephson effect describes the generic appearance of a supercurrent in a weak link between two superconductors. Its exact physical nature however deeply influences the properties of the supercurrent. Detailed studies of Josephson junctions can reveal microscopic properties of the superconducting pairing (spin-triplet correlations, $d$-wave symmetry) or of the electronic transport (quantum dot, ballistic channels). In recent years, considerable efforts have focused on the coupling of superconductors to topological insulators, in which transport is mediated by topologically protected Dirac surface states with helical spin polarization (while the bulk remains insulating). Here, the proximity of a superconductor is predicted to give rise to unconventional induced $p$-wave superconductivity, with a doublet of topologically protected gapless Andreev bound states, whose energies varies $4\pi$-periodically with the superconducting phase difference across the junction. In this article, we report the observation of an anomalous response to rf irradiation in a Josephson junction with a weak link of the 3D topological insulator HgTe. The response is understood as due to a $4\pi$-periodic contribution to the supercurrent, and its amplitude is compatible with the expected contribution of a gapless Andreev doublet.